1. Field of the Invention
The present invention generally relates to thermoplastic polymers utilized in small diameter tubing for catheter bodies. More particularly, the present invention relates to polymer blends useful for forming catheter bodies having improved torque transmitting properties for easy maneuverability through tortuous body pathways yet having sufficient compliance to avoid tissue perforation.
2. Description of Related Art
Thermoplastic polymers, copolymers, and polymer blends have been used extensively in the fabrication of medical devices, including a wide range of long term and short term implant devices. Many polymers and polymer blends used in medical devices have specific physical and chemical properties which make them particularly suitable for in vivo applications. Preferred chemical, physical and thermomechanical properties depend upon the specific function, the type of tissue, cells or fluids contacting the medical device and the acceptable or desired manufacturing processes. Major considerations in choosing polymers for medical devices include the chemical stability of the polymer, particularly hydrolytic stability, the toxicity of the polymer, and the degree of interaction between tissue or blood and the polymer. Additionally, the polymer or polymer blend should meet all the physical demands relating to the function of the medical device including strength, compliance, stiffness, torque and rebound properties. Also, it should be compatible with and contribute to stable manufacturing and assembly processes.
Catheters represent a particularly large class of medical devices used for a variety of in vivo applications. Typically catheter bodies are formed of one type of polymer, but more than one type can be incorporated into the catheter body in order to provide a device which meets the catheter's physical and chemical requirements. One type of catheter which is widely utilized in a variety of procedures is physically designed to be maneuvered through tortuous fluid pathways within a body to a preselected site. Once maneuvered into place the catheter and its components are used to assist in the performance of a variety of monitoring functions such as cardiac output monitoring, pulmonary artery pressure monitoring and blood chemistry analyses. The catheter is frequently allowed to remain for a period of several days inside the body while liquids and medicaments are infused through its lumens and into the fluid pathway.
In order to safely maneuver the catheter into place, the material used to fabricate the catheter body should have sufficient rebound and low enough bend stiffness to avoid perforating or otherwise harming vessel endothelial tissue. That is, the material should have such combinations of mechanical properties as to allow the catheter to move away from the vessel wall without damaging it, should it inadvertently collide with the wall. At the same time, the material should have good torque transmitting properties to provide catheter maneuverability.
Torque transmitting properties are particularly important when a catheter is being introduced in the heart via the inferior vena cava. When introduced into the right heart, the catheter distal tip must first turn in one direction to pass through the tricuspid valve and then do an abrupt turn in order to traverse the right ventricle and enter the pulmonary artery. During this process, the proximal end of the catheter is frequently subjected to a constant torque in order to maneuver the catheter distal tip to the selected site.
When subjected to constant torque at the proximal end, the distal end of catheters fabricated of some materials will seem to be stuck at one point and then rapidly "whip" to another position. This "whipping" action can cause serious tissue damage or tissue perforation. Hence, optimum catheter materials should be designed to provide enough maneuverability, yet reduced chance of harmful "whipping".
Catheter torque transmissibility should be maintained at both ambient temperatures and body temperature because during the insertion process a large part of the catheter is external to the body and remains at room temperature. At the same time, the distal end of the catheter reaches body temperature and if the torque decreases substantially at the elevated temperature, the catheter maneuverability will be unacceptable.
Plasticized polyvinylchloride has been used widely to form a number of types of general purpose catheters. The torque transmitting properties of this polymer are good at room temperature. Plasticized polyvinylchlorides however, exhibits a significant decrease in torque at body temperatures, when ideally the torque should be still high enough to facilitate maneuverability once the portion of the catheter in the body warms to body temperature. Thus, polyvinylchloride catheters fall short of having ideal torque transmitting properties.
In an attempt to improve the maneuverability of these catheters, some manufacturers have incorporated braids of suitable materials into the walls of extruded catheters. These braids have the effect of increasing the torque transmitting characteristics without severely compromising the flexibility of the catheter.
A major disadvantage associated with using braids is that the functional demands of these catheters have increased to the point that 7F catheters with 5 lumens are common. These high performance catheters are frequently coated with a heparin complex so that they can remain within a blood vessel for several days without causing excessive platelet formation. During this time all lumens may be utilized to perform a variety of functions. In order to accommodate 5 lumens within a single catheter, the catheter walls and the lumen walls must be very thin which necessarily leaves little volume for incorporating braids within the walls.
Another disadvantage associated with polyvinylchloride catheters involves the public concern, particularly in European countries, regarding the use of this polymer. This concern relates to environmental disposal problems associated with PVC. Additionally, polyvinylchloride catheters require substantial levels of plasticizer in their material formulation and the use of these plasticizers present various challenges. The plasticizers tend to migrate to the surface, becoming a potential problem to neighbor components which may absorb them. Also, as the plasticizer content changes the thermomechanical properties of the part change. This limits the shelf life of the product.
During the past decade researchers have learned a great deal about the interaction of materials with blood and tissue. Studies have indicated that certain polymers are much more compatible with blood and tissue in that they cause less platelet formation and less severe inflammatory response when implanted in living organisms. While, polyvinylchloride catheters have been used extensively by medical professionals, polyvinylchloride is not known to have exceptional biocompatibility. On the other hand, polyurethanes have recently gained recognition for their biological compatibility.
Accordingly, it is an objective of the present invention to provide a polymeric composition which can be used to fabricate catheters having good torque transmission properties at ambient temperatures and at body temperatures, and having sufficient softness and flexibility to avoid tissue trauma.
It is additionally an objective of the present invention to provide a polymeric composition having improved blood and tissue compatibility.
It is another objective of the present invention to provide a polymeric composition which is homogeneous at its melt temperature and easily processed into small diameter tubing.
It is another objective of the present invention to provide a polymeric composition for fabricating into catheters having surfaces which will accept coatings of heparin complexes.